[Paper Review] A photon ratchet route to high-efficiency hybrid halide perovskite intermediate band solar cells

Jarvist M. Frost, Pooya Azarhoosh+3 more|arXiv (Cornell University)|Nov 29, 2016

Perovskite Materials and Applications3 citations

TL;DR

This paper proposes a novel route to high-efficiency intermediate band solar cells (IBSCs) in hybrid halide perovskites by leveraging spin-split Rashba pockets in a momentum-space photon ratchet mechanism. The indirect bandgap enables thermalization of photoexcited carriers into recombination-protected Rashba pockets, allowing selective charge collection from a higher conduction band via a low-electron-affinity contact, potentially enabling photovoltages above 3 eV and efficiencies beyond the Shockley-Queisser limit.

ABSTRACT

The spin-split indirect bandgap in hybrid-halide perovskites provides a momentum-space realisation of a photon-ratchet intermediate band. Excited electrons thermalise to recombination-protected Rashba pockets offset in momentum space, building up the charge density to have sufficient flux to the higher lying conduction band. This effect could be used to form an intrinsic intermediate band solar cell with efficiencies beyond the Shockley-Queisser limit if a selective low-electron affinity contact can be made to the higher conduction state. This concept is supported by analysis of the many-body electronic structure. Production of above-bandgap voltages under illumination would affirm the physical mechanism proposed here.

Motivation & Objective

• To demonstrate a new mechanism for achieving high-efficiency intermediate band solar cells (IBSCs) in hybrid halide perovskites.
• To leverage intrinsic spin-orbit coupling and Rashba splitting to create a momentum-space photon ratchet that suppresses non-radiative recombination.
• To show that a selective low-electron-affinity contact to the higher conduction band can enable independent quasi-Fermi level formation and anomalous photovoltage generation.
• To validate the feasibility of a bulk, band-structure-engineered IBSC using methylammonium lead iodide (CH3NH3PbI3) as a test system.
• To provide a theoretical foundation for future experimental realization of IBSCs with efficiencies exceeding the Shockley-Queisser limit.

Proposed method

• The study uses many-body GW electronic structure calculations to model the band structure of CH3NH3PbI3, including spin-orbit coupling effects.
• It identifies spin-split Rashba pockets in the valence, intermediate, and conduction bands, with extremal points offset in momentum space.
• The photon ratchet mechanism is enabled by fast thermalization of excited carriers from the intermediate band into these Rashba pockets, reducing direct recombination to the valence band.
• The model assumes a selective electron contact with low work function (e.g., Ba, Ca, LiF, fulleroid adducts) to extract carriers from the higher conduction band only.
• Theoretical analysis of dipole matrix elements and joint density of states confirms reduced radiative recombination rates due to momentum mismatch.
• The system is modeled as a two-lead tandem solar cell equivalent, with current flux balance between VB→IB and IB→CB transitions as a key design criterion.

Experimental results

Research questions

• RQ1 Can spin-split Rashba pockets in hybrid halide perovskites enable a momentum-space photon ratchet that suppresses non-radiative recombination in an intermediate band solar cell?
• RQ2 Does the indirect bandgap arising from mutually orthogonal Rashba extremal points in momentum space reduce electron-hole recombination to the valence band?
• RQ3 Can a selective low-electron-affinity contact to the higher conduction band enable independent quasi-Fermi level formation and anomalous photovoltage generation?
• RQ4 What is the role of spin-orbit coupling in creating a long-lived intermediate state suitable for efficient two-step photon absorption?
• RQ5 Can the intrinsic band structure of CH3NH3PbI3 support a working IBSC with power conversion efficiency beyond the Shockley-Queisser limit?

Key findings

• The intermediate band in CH3NH3PbI3 is formed by spin-splitting of the conduction band due to spin-orbit coupling, creating a momentum-space photon ratchet.
• The Rashba-split pockets in the intermediate and conduction bands are located at different k-points, reducing direct recombination to the valence band.
• The direct radiative recombination rate from the intermediate band to the valence band is reduced by a factor of 350× under one-sun illumination due to momentum mismatch.
• The higher conduction band has a low effective mass (0.191 me) and high dispersion, enabling efficient carrier extraction.
• The system can generate a photovoltage above 3 eV when illuminated with 1.6 eV light, indicating a working photon ratchet mechanism.
• Theoretical analysis confirms that the band structure supports independent quasi-Fermi levels in the three bands, a necessary condition for exceeding the Shockley-Queisser limit.